1. Field of the Invention
The present invention relates to an apparatus for detecting the modulation rate of voice-band data signals. Three measures are used to determine the data rate. These are the level occupancy, L.sub.p, the partial entropy, E.sub.p, and the zero-crossings, N.sub.zc. By first normalizing the signal level and then using predetermined thresholds for each of these measures, the data rate can be determined with a high degree of accuracy.
2. Description of the Prior Art
There is an increasing need to carry high speed data over voice channels. Recently, Low Rate Encoders (LRE) operating at 32 kbit/s or lower rates have been used with increasing frequency to transmit speech signals. However, most 32 kbit/s LRE's cannot support transmission of data signals modulated at rates greater than 4800 bps without introducing high bit error rates (BER greater than 10.sup.-3). Even at data rates equal to 4800 bps, the performance of 32 kbit/s LRE's is sometimes only marginal (BER=10.sup.-5) depending on the analog impairments on the line. Therefore, it may be necessary to use higher transmission rates such as 40 kbit/s LRE rate or 64 kbit/s PCM to carry data signals whose modulation rates are equal to or greater than 4800 bps. A Data Rate Detector (DRD) may be required so that an adequate LRE transmission rate can be selected to carry a given data rate.